There are three types of earthquakes that occur on land in the Kanto region: first, shallow earthquakes (depths of 0-20 km) that occur on active faults; second, somewhat deeper earthquakes (depths of 20-50 km) that occur on the surface of or inside the subducting Philippine Sea Plate; third, deep earthquakes (depths of 50-100 km) that occur on the surface of or inside the subducting Pacific Plate. Thus, a characteristic of the Kanto region is that it has several seismically active areas under the landmass (Fig.5-9). Also, the area from western Kanagawa Prefecture to eastern Yamanashi Prefecture corresponds to an extension of the Sagami Trough on land that is the boundary between the Philippine Sea Plate and the plate on land. Earthquakes here are generated by complicated forces that accompany the collision with the Izu Peninsula.

1) Shallow inland earthquakes (Depths of 20 km or less)

The topography of the Kanto region is characterized by the broad Kanto Plain surrounded by mountainous and hilly areas, including the Kanto Mountains (Fig.5-7).

The primary active faults in the Kanto region are located primarily in the south. In particular, many highly active faults (A-B classes) are located in the southern part of the Boso Peninsula, the southern tip of the Miura Peninsula, and western Kanagawa Prefecture. These active faults are thought to be closely related to the Philippine Sea Plate that is subducting from the Sagami Trough. It is possible that their activity occurs together with the interplate earthquakes along the Sagami Trough. The most active fault is the Kannawa/Kouzu-Matsuda fault zone that lies from the northwest to the southeast in western Kanagawa Prefecture. It has an A class activity. This fault zone extends as a continuation of the submarine active fault in the Sagami Bay. The active fault zone in the southern Boso Peninsula and the southern tip of the Miura Peninsula also probably extend to sea.

Several active faults lying from northwest to southeast whose activity is classified as B and C classes are known to exist at the border of the Kanto Mountains and the Kanto Plain. These include the northwestern margin of the Kanto-Plain (Kanto-heiya-hokuseien) fault zone and the Tachikawa fault zone. Some areas in the Kanto Plain have been covered with thick sediment from a relatively recent period known as the Quaternary Period (starting about 2 million years ago). Underneath this sediment is an older stratum of bedrock that continues from the surrounding mountainous area. The surface of this bedrock forms a large valley whose depth reaches 4,000 m in places. This subterranean valley extends in a northwest-southeast direction to pass through the Kanto Mountains and the Ashio Mountains (Asio Santi). It is assumed that several active faults classified as B and C class activity exist as an extension of this valley, stretching in a northwest to southeast direction from the central Kanto Plain to the coast of Tokyo Bay. These faults include the Arakawa fault and the northern margin of the Tokyo Bay (Tokyo Wan-hokuen) fault . These active faults are covered by sediment from a relatively recent period, and their shape will be better understood through surveys of the underground structure. It is possible that undiscovered active faults lie under the Kanto Plain. From the extent of topographical deformation in the surface, however, it is unlikely that any of these faults would have an activity of an A class. While there are few active faults in the northern Kanto Plain, the Sekiya fault with an A class activity is in northern Tochigi Prefecture.

Considering the direction and type of activity of the active faults in the Kanto region, the direction of the force exerted on these active faults differs depending on the location. This is believed to result from the complex forces coming from different directions under the Kanto region due to the influence of the subducting Philippine Sea Plate and the Pacific Plate.

Several destructive earthquakes are known to have occurred at shallow locations on land. Since the Meiji period (1868), these have included the 1931 West Saitama Earthquake (M 6.9) and the 1949 Imaichi earthquakes (M 6.2 and M 6.4). Previously there have been the 818 earthquake in Kanto Provinces (larger than M 7.5), the 878 Sagami-Musashi Earthquake (M 7.4), and the 1683 earthquake near Nikko (M 7.0). The possibility exists that the 878 Sagami-Musashi earthquake took place on the Isehara fault, and the 1683 earthquake near Nikko on the Sekiya fault. All of these earthquakes, however, do not correspond to known active faults. Further, earthquakes of M 6 or so that cause local damage sometimes occur even outside the active fault zone. Also, the intervals of activity for most active faults are 1,000 years or more. This does not indicate, however, that no earthquakes can occur here even if none are known at present.

Regarding the 1855 Ansei Edo Earthquake (M 6.9), there are two ideas: first, it has been caused by active fault movement close to the surface; second, it has occurred on the upper surface of the Philippine Sea Plate.

Frequent seismic activity regularly occurs in the Nikko and Ashio areas of Tochigi Prefecture, as well as the area surrounding the Izu Islands. This seismic activity sometimes causes damage locally.

2) Intermediate-depth inland earthquakes (Depths of 20-50 km)

One of the earthquakes that occurred on the surface of the subducting Philippine Sea Plate was the 1968 earthquake in central Saitama Prefecture (M 6.1; roughly 50 km deep). Brisk seismic activity at a depth of roughly 50 km is also known to occur regularly in the western part of the two seismically active areas in southwest Ibaraki Prefecture (part a of Fig.5-4 and Fig.5-5). Earthquakes also occur in the interior of the Philippine Sea Plate. Examples include the 1987 earthquake near Kujukuri Beach (Kujukuri-hama) on the Boso Peninsula (M 6.7; depth of roughly 50 km: This is also referred to as the Earthquake of the East Coast of Chiba Prefecture.)

Regarding the 1855 Ansei Edo Earthquake (M 6.9), there are two ideas: first, it has been caused by active fault movement close to the surface; second, it has occurred on the upper surface of the Philippine Sea Plate.

3) Deep inland earthquakes (Depths of 50-100 km)

Brisk seismic activity at a depth of roughly 70 km is known to occur regularly as earthquakes on the surface of the subducting Pacific Plate in the eastern part of the two seismically active areas in southwest Ibaraki Prefecture (Fig.5-4 and Fig.5-5). Brisk seismic activity from the surface to the interior of the Pacific Plate is also known to occur at a depth of roughly 80 km in northwest Chiba (Tiba) Prefecture (Fig.5-4 and Fig.5-5). Recent earthquakes known to have occurred within the Pacific Plate are the 1985 earthquake in southern Ibaraki Prefecture (M 6.1; depth of roughly 78 km), the 1988 earthquake in the eastern part of the Tokyo Metropolitan District (M 6.0; depth roughly 96 km), and the 1992 earthquake in the southern part of Tokyo Bay (near the Uraga Channel; M 5.9; depth roughly 92 km). The depth of the 1894 earthquake known as the Meiji Tokyo Earthquake (M 7.0) has not been determined, but it is thought to have occurred within the subducting Pacific Plate under the Kanto region.

4) Earthquakes in western Kanagawa Prefecture

There has been a series of destructive earthquakes with M 7 or so in the area from western Kanagawa Prefecture to eastern Yamanashi Prefecture. These include the 1633 Sagami-Suruga-Izu Earthquake (M7.0), the 1782 Sagami-Musashi-Kai Earthquake (M 7), and the 1853 earthquake near Odawara (M6.7). The plate structure in this area is complex, and it is not possible to determine in a general way if these earthquakes occurred between plates or within plates. Also, the Kannawa/Kouzu-Matsuda fault zone with an A class activity is located at the surface here. Its relationship to the boundary of the subterranean plate has already been pointed out.

A look at recent seismic activity shows that brisk activity occurs regularly at a depth of 10-30 km from the Tanzawa Mountains (Tanzawa Santi) to eastern Yamanashi Prefecture. There are frequent earthquakes in the M 5-6 range that cause some damage. It is also possible for a tsunami to accompany these earthquakes when the source region extends into the sea.